1. Field of Invention
The present invention relates in general to power converters, and more particularly, to the control circuit for switching power converters.
2. Description of Related Art
Power converters have been used for converting an unregulated power source to a regulated voltage and/or current source. The control circuit of the power converter generates a switching signal for the power regulation. The duty cycle of the switching signal is modulated in accordance with the output of the power converter. The switching frequency of the switching signal generally is self-determining in the control circuit of the power converter. To reduce the switching noise and the EMI (electrical and magnetic interference) while more than one switching power converters are used in a system, the synchronization of the switching signal is required. However, in recent developments, many PWM (pulse width modulation) control circuits have been proposed for power converters for saving power losses, such as “PWM controller having off-time modulation for power converter” by Yang, U.S. Pat. No. 6,545,882; “PWM controller having a modulator for saving power and reducing acoustic noise” by Yang, et al, U.S. Pat. No. 6,781,356. The switching frequency of the above prior arts is varied in response to the change of the load, which causes difficulties for the control circuit to synchronize the switching signal with other power converters. Besides, the fast synchronization of the switching signal is required. A slow phase locking of the switching frequency will generate jitters and acoustic noises in the power supply. Furthermore, due to the variability in load conditions, the synchronization of the switching signal may cause an unstable supply voltage during the turning-on of the power converter.